A cover for a metallurgical crucible in which metal is melted or treated in molten condition normally must be cooled to protect it from the extremely high temperatures reigning in the vessel or crucible. Accordingly it is standard to make such a cover as an annular and tubular outer ring or collar supporting an array of cooling tubes and to circulate a coolant such as water or steam through the collar and tubes.
It is frequently necessary to provide one or more holes in the cover so that a lance or electrodes can be inserted through it, and so that gases can be drawn out of the crucible through the cover. In an electric-arc device in particular it is necessary to provide three such openings for the electrodes of standard three-phase system. These electrodes heat up a great deal themselves, so it is standard practice to provide relatively large openings formed by substantial metallic rings in good heat-transmitting engagement with the cooling tubes, and to use separate cooled jacket on each electrode. Thus each electrode with its own cooling device is fitted on the respective ring.
It has also been suggested in German patent document No. 2,917,755 (British Pat. No. 2,048,445) to use a refractory center piece in the cover, and to form it with the necessary electrode holes. Such an arrangement, although in principle substantially simpler than the other above-described system, has a relatively short service life because the ceramic insert and the metallic cover have different coefficients of thermal expansion. Thus they work against each other, normally with the result of cracking the insert.